| 摘要 |
An optical scanning device scans a surface (2) provided with marks with a focused radiation beam (6) along a scan line. The device has a radiation-sensitive detection system (8) divided in two detectors (A,D; B,C) along a dividing line (Y), which is perpendicular to the scan line and which intersects the axis (14) of the radiation beam coming from the surface. A focus error signal is formed by taking the time or phase difference between corresponding parts of the detector signals relating to passage of the radiation beam over patterns of the marks having a spatial frequency in a range from 0.14 to 0.2 times a cut-off frequency of the objective system and in a range from 0.35 to 0.5 times the cut-off frequency. |
